In yeast, the plasma membrane $Na^+/H^+$ anti porter and $Na^+$-ATPase are key enzymes for salt tolerance. Saccharomyces cerevisiae $Na^+$-ATPase (Ena1p ATPase) is encoded by the ENA1/PMR2A gene; expression of ENA1 is tightly regulated by $Na^$ and depends on ambient pH. Although Ena1p is active mainly at alkaline pH values in S. cerevisiae, no $Na^+$-ATPase has been found in flowering plants. To test whether this yeast enzyme would improve salt tolerance in plants, we introduced ENA1 into Arabidopsis (cv. Columbia) under the control of the cauliflower mosaic virus 35S promoter. Transformants were selected for their ability to grow on a medium containing kanamyin. Southern blot analyses confirmed that ENA1 was transferred into the Arabidopsis genome and northern blot analyses showed that ENA1 was expressed in the transformants. Several transgenic homozygous lines and wild-type (WT) plants were evaluated for salt tolerance. No obvious morphological or developmental differences existed between the transgenic and WT plants in the absence of stress. However, overexpression of ENA1 in Arabidopsis improved seed germination rates and salt tolerance in seedlings. Under saline conditions, transgenic plants accumulated a lower amount of Na+ than did the wild type, and fresh and dry weights of the former were higher. Other experiments revealed that expression of ENA1 promoted salt tolerance in transgenic Arabidopsis under both acidic and alkaline conditions.